Studies on the purification and characterization of cyclic AMP-dependent protein kinases from bovine brain are in progress. Also, attempts are being made, primarily by affinity chromatography, to separate the regulatory and catalytic subunits in these enzymes. An important objective of this work is to delineate the mode of cyclic AMP binding and the nature of the amino acids in the cyclic AMP binding sites of the kinases. From this information it may be possible to develop a detailed understanding of the way in which cyclic AMP stimulates the catalytic activity of an isolated enzyme system.